Agree about the yellow flag iris - mine was planted as a marginal but obviously doesn't understand the meaning of the word (I have had to use a pruning saw to cut back the roots of mine as they creep stealthily across the water, which is both tricky and a bit bum-clenching in a PVC lined pond! )

A water lily can look very attractive, but make sure you choose one of an appropriate size for your pond's size and depth.

Of the submerged oxygenating plants I've found hornwort the most reliable, and of quite pleasing appearance too.